1. Field of the Invention
This invention relates to printheads for continuous stream-type ink jet printers and more particularly to such printheads having an elongated droplet generator from which a plurality of equally spaced ink streams are emitted, the ink streams being stimulated by traveling sound wave generated at one end of the droplet generator.
2. Description of the Prior Art
Ink jet devices of the continuous stream type generally employ a printhead having a droplet generator with multiple nozzles from which continuous streams of ink droplets are emitted and directed to a recording medium or a collecting gutter. The ink is stimulated prior to or during its exiting from the nozzles so that the stream breaks up into a series of uniform droplets at a predetermined distance from the nozzles. As the droplets are formed, they are selectively charged by the application of a charging voltage by electrodes positioned adjacent the streams at the location where they break up into droplets. The droplets which are charged are deflected by an electric field either into a gutter for ink collection and reuse or to a specific location on the recording medium, such as paper, which may be continuously transported at a relatively high speed across the paths of the droplets.
Printing information is transferred to the droplets through charging by the electrodes. The charging control voltages are applied to the charging electrodes at the same frequency as that which the droplets are generated. This permits each droplet to be individually charged so that it may be positioned at a distinct location different from all other droplets or sent to the gutter. Printing information cannot be transferred to the droplets properly unless each charging electrode is activated in phase with the droplet formation at the associated ink stream. As the droplets proceed in flight towards the recording medium, they are passed through an electric field which deflects each individually charged droplet in accordance with its charge magnitude to specific pixel locations on the recording medium.
A common method of perturbating an array of continuous ink streams is by a piezoelectric driver which produces acoustic waves which traverse an ink reservoir to the nozzles, perturbating the ink streams and ideally causing uniform breakup of the streams in terms of break off length. Thus, the drop generator reservoir to manifold has two functions, to distribute ink to the individual nozzles and to distribute acoustic energy to the individual streams to cause a controlled uniform breakup into droplets.
Since the reservoir is an acoustic pathway to the ink streams, it must be acoustically designed. This means the materials used should be acoustically matched to the ink and the fabrication must be of high precision. The completed droplet generator must have a piezoelectric driver accurately positioned in a precision reservoir confronting a precise array of nozzles. The droplet generators successfully meeting the design criteria tend to be quite bulky and heavy and costly to fabricate. Further, the design of a long array, multiple stream droplet generator usually presents a formidable problem in achieving uniform stimulation of the stream breakup because of standing wave formation. Non-uniform stimulation in a long array creates many disadvantages and difficulties in ink jet printing. For example, the jets will break up in a wide range of break off lengths, necessitating a very long charge tunnel. Also non-uniform stimulation will cause the ink streams to break up in different satellite conditions that complicates the charging mechanism. All liquid jet streams which are stimulated with low acoustic power normally break up into a series of uniform, large drops separated by much smaller drops called satellites. The satellite droplet separates first from the jet stream in front of the main droplet, which then separates next. Later, the satellite merges backward into the main drop. This is called the rear merge condition. Because the two droplets are formed at different times, they may be exposed to the same or different charging voltages depending on the charging phase relationship to the droplet break off phase. In both cases, the droplets can be charged and deflected to their targets accurately. However, a typical charging phase window for a rear merge condition is about 200 degrees, which is marginal for a practaical ink jet printer. Depending on the power level of stimulation, the large drops and accompanying satellites fall into various categories of rear merging satellite, forward merging satellite, and no satellite. In the cases of forward merge satillite and no satellite conditions, the charge phase window is about 300 degrees, which provides the necessary latitude for phase drift. Generally speaking, it is critical for all jet streams in a multiple jet printer to have the droplets break off in a forward merge or no satellite condition if high quality printing and a reliable printing system is to be achieved.
U.S. Pat. No. 3,683,212 to Zoltan discloses a pulse or on-demand droplet ejecting system having a tubular piezoelectric member through which the liquid ink flows in route to the nozzle through which the droplets are ejected. An electrical pulse applied to the transducer creates an acoustical pressure pulse caused by the periodic constricting of the piezoelectric member. Each constriction ejects a droplet.
U.S. Pat. No. 3,739,393 to Lyon et al discloses a traveling wave drop generator wherein one end of the nozzle plate is vibrated to propagate the traveling wave. This causes a continuing series of bending waves to travel the length of the nozzle plate. Ink stream stimulation by prior art methods are illustrated in FIG. 5A of the patent to Lyon et al, while the stimulation in accordance with the Lyons et al invention is shown in FIG. 5B. The prior art ink stream broke off at varying distances from the nozzles necessitating an elongated charging electrode and the attendant printing quality impact as such elongated charging electrodes caused. In contrast to the prior art break off lengths in FIG. 5A, the stream stimulated in accordance with Lyon et al have a nearly uniform nominal stream length. The only significant variation being a slightly longer length of ink stream displaced along the plate in the direction of the bending travel wave. This lengthening is the predictable result of bending wave attenuation during propagation and the relatively small printing errors associated therewith may be corrected by introducing fixed time delays in the charge circuits.
U.S. Pat. No. 4,554,558 to Beaudet et al discloses a fluid jet printhead comprising a plurality of piezoelectric means which when electrically excited produce pressure waves which travel through the ink. An acoustic isolation material made of a polyurethane foam surrounds the piezoelectric means. However, Beaudet et al does not use a traveling wave droplet generator.
U.S. Pat. No. 4,523,202 to Gamblin discloses the use of a random signal generator to drive an electroacoustic transducer that is coupled to the ink so as to reduce the adverse the printing effects otherwise caused by standing acoustic waves along the length of an orifice array. The Gamblin ink jet printer also does not use traveling wave drop generator.